1. Field of the Invention
This invention relates to an improvement in air-operated combination diaphragm spring brake actuators of the type used with air brake systems on vehicles such as trucks. In one of its aspects, the invention relates to a spring brake actuator having a tamper-resistant spring chamber.
2. State of the Prior Art
Spring-applying brake actuators are in common use with air brake systems used on trucks, buses, and towed vehicles. Such actuators are normally provided with a service chamber for normally applying and releasing the brakes in response to delivery and exhaust of compressed air, and a spring chamber disposed in tandem with the service chamber for providing parking or emergency brake functions. A spring brake actuator uses spring force to operate a service brake actuator and apply brakes when pressurized air in the spring chamber is reduced below some predetermined level. Air pressure may be reduced in the spring chamber to apply the brakes under the control of the operator or automatically as a result of failure of the air system. The service chamber and spring chamber are separated by an adapter or flange casing which forms a wall between the two chambers.
In a typical spring brake, a barrel-shaped power spring is used to store energy and to exert the large force required for braking in the event of air pressure failure. Air pressure acting on a diaphragm is employed to compress the spring and maintain it in its brake release position. When the air is exhausted, the spring acts on the diaphragm, typically a elastomeric diaphragm or a piston, and through an actuating rod exerts the spring force on the service push rod to apply the brakes in the event of a failure of the system air pressure.
The spring brake actuator operates within the spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and the adapter. The power spring is typically compressed within the spring chamber between the head and the diaphragm. The spring has a high spring constant and typically weighs 3 pounds or more, being compressed to a linear length of less than 3 inches from an original uncompressed length in an extended condition of from 9 to 12 inches. With a high spring constant, the spring has a substantial amount of potential energy, exerting a force on the head of from 2,000 to 3,000 pounds.
Various approaches have been heretofore proposed for sealing the brake actuator head to the adapter to prevent disassembly of the two parts. In U.S. Pat. No. 4,565,120, a safety restraint comprises a sheet metal body having a notch which receives the flanges on the adapter. The notch is large enough to permit partial separation of the head from the adapter upon release of the clamp band. In U.S. Pat. No. 4,850,263, an annular lip on the head is spun over the flange on the adapter to capture and grip it. The head can be separated from the adapter only by deforming the lip. In Gummer et al. U.S. Pat. No. 4,960,036, a circular ring, U-shaped in cross section, partially surrounds an adapter flange and a head flange and is crimped at the ends to retain the ring in place.